1. Field of the Invention
The present invention is in the field of methods and apparatus for flow injection analysis of total inorganic phosphate in aqueous systems. Inorganic phosphates such as orthophosphate and polyphosphate are products which are used in aqueous systems such as cooling towers and boilers to prevent calcium scales and corrosion. There is a threshold concentration at which phosphates work by causing crystal distortion that prevents calcium scale and/or steel corrosion. Thus, it is important to determine on a regular basis the concentration of dissolved inorganic phosphates so that the threshold levels necessary to prevent deposition of calcium scales is maintained. A secondary goal is to prevent high phosphate concentrations which may result in subsequent phosphate scale.
However, the analytical measurement of these inorganic phosphates is important in other fields such as environmental, clinical and agricultural analysis, where the determination of inorganic phosphate levels can also be important, as, for example, the determination of the phosphate levels in fertilizers or in clinical specimens such as plasma. The novel method of the present invention is applicable to these areas as well.
Flow injection analysis (FIA) is a well-known, simple and reliable technique based on continuous flow of a sample solution which is introduced directly into an unsegmented carrier stream of a reagent solution, thereby forming a well-defined sample zone. In so called reverse flow injection analysis, the reagent is introduced into an unsegmented sample stream. While it is being transported to a detector device further downstream, the sample has an opportunity to react with the reagent and form a new chemical species which can be quantitatively measured by the detector. The reaction is usually a color-forming one and the detector, therefore, a colorimeter (spectrophotometer). FIA lends itself to the automated, rapid and reliable analysis of various samples, and offers many advantages over the older technique of air-segmented continuous flow analysis.
Apparatus means for carrying out a typical FIA method will depend upon the reaction being carried out, but elements which are almost always present include tubing through which the samples and reagents are carried, means for moving said samples and reagents through said tubing and means for mixing them together, reagent reservoirs, means for collecting, filtering and supplying samples, heating, and analyzer means such as a colorimeter.
The color-forming reaction which has been used in the past to provide a means for quantitatively analyzing inorganic phosphate content is the well-established one wherein a mixed solution of molybdenum (V) and molybdenum (VI) reacts with orthophosphate to produce the heteropoly blue complex. It is well known that orthophosphate (P.sub.1) reacts with a molybdenum (VI) reagent to form a yellow heteropoly complex and that subsequent reduction of the yellow complex by ascorbic acid or other suitable reductants gives heteropoly blue complex containing Mo(V) and Mo(VI). The formation of these heteropoly complexes has been extensively applied to the determination of phosphorus by flow injection analysis and by air-segmented flow analysis.
The total inorganic phosphate content of a sample to be analyzed will usually never be all orthophosphate; however, so that the color-forming reaction described above cannot be utilized. It is necessary to first convert the various types of inorganic phosphates present to orthophosphate in order to proceed with the flow injection analysis. Typically, the inorganic phosphates which are not orthophosphates are polyphosphates, which have the general formula M.sub.x+2 P.sub.x O.sub.3x+1, and include, e.g., pyrophosphate (diphosphate, P.sub.2) and tripolyphosphate (triphosphate, P.sub.3). These polyphosphates may be converted to orthophosphate by hydrolysis using concentrated sulfuric acid or other inorganic acids at high temperatures (&gt;100.degree. C.) and pressures for a suitable period of time, in accordance with well known procedures employed in FIA.
Typically, the acid hydrolysis reagent, e.g., concentrated sulfuric acid, is combined with the color-forming reagent, i.e., the Mo (V and VI), by dissolving the latter in the former. After the inorganic phosphates are converted to orthophosphate with this combined reagent at high temperature and pressure, and the yellow heteropoly complex is formed, in a subsequent step ascorbic acid or other reducing agent is added to the reaction mixture to form the heteropoly blue complex, which is then measured on a colorimeter.
Of course, it is also possible to use the method of the present invention to provide for the flow injection analysis of orthophosphate only, if that is desired. This may be accomplished using the same method as for polyphosphate, except that during the step of heating the reaction mixture to convert polyphosphate to orthophosphate, the reaction temperature is maintained at a lower level, sufficient for the color-forming reaction to proceed.
2. Brief Description of the Prior Art
Elemental analysis of inorganic phosphates using methods and devices other than flow injection analysis are well known; see, e.g. U.S. Pat. Nos. 3,137,543; 3,846,074; and 4,836,773.
Hirai et al., Anal. Chim. Acta, 115, 269-277 (1980), describe a flow injection analysis method for inorganic polyphosphates, but employ high temperatures (140.degree. C.) and pressures (5 kg cm.sup.-2 =@ 70 psi). A solution of 0.1M L-ascorbic acid containing 50 mL of acetone is employed, but this is in a segmented flow analysis method and is used as a reducing agent for the molybdenum reagent in the color-forming step. None of this disclosure suggests the use of the ascorbic acid and acetone as a carrier stream for the sample in a flow injection analysis method as in the present invention, with the surprising advantages of lower temperature and pressure for the step of hydrolysis conversion of the polyphosphates to orthophosphate.
Hirai et al., J. Chromatogr., 206, 501-509 (1981), describe a flow injection analysis method in which lower oxo acids of phosphorus such as phosphinate and phosphonate may be determined in addition to orthophosphate by oxidizing them in a solution of sodium hydrogen sulfite and molybdenum(V)-molybdenum(VI).
Yoza et al., Anal. Chim. Acta, 121, 281-287 (1980), describe a flow injection analysis method for the determination of polyphosphates [but only pyrophosphate (P.sub.2) and tripolyphosphate (P.sub.3), whereas the method of the present invention can also determine hexametaphosphate (P.sub.6)] which can be carried out at room temperature because it does not involve hydrolysis conversion of the polyphosphates to orthophosphate. Quantitative determination is made by measurement of the u.v.-absorption of colored metal complexes of xylenol orange and methylthymol blue with the polyphosphates.
Fogg et al., Analyst, 108, 1485-1489 (1983), describe the effect of increasing ethanol and acetone concentrations on the differential-pulse voltammograms used in flow injection voltammetric determination of total phosphate at a glassy carbon electrode. This method uses manual digestion of polyphosphates, and further, would not be readily adaptable to process analysis.
Motomizu et al., Talanta, 30, 333-338 (1983), describe a flow injection analysis method for the determination of trace amounts of phosphate in river water using a reaction with molybdate and Malachite Green in acidic medium to form a green species. This method measures orthophosphate only at trace levels, and does not measure polyphosphates.
Baba et al., J. Chromatogr., 295, 153-160 (1984), describe a parallel detection flow injection system for the simultaneous determination of phosphate and phosphonate. This method uses high pressure digestion and dual detection; whereas, the method of the present invention uses only a single channel, and is therefore less expensive to operate.
Yoza et al., J. Chromatogr., 325, 385-393 (1985), describe a flow injection analysis method for determination of inorganic polyphosphates wherein they are hydrolyzed by inorganic pyrophosphatase before reaction with a molybdenum(VI) reagent for colorimetric determination. This method is applicable only to P.sub.3, P.sub.2, and P.sub.1 species; and enzymes are unstable and would not lend themselves to continuous on-line process analysis.
Pauer, et al., Water SA, 14, 125-130 (1988), describe a flow injection analysis method for low concentrations of phosphate. The method uses tin chloride as a reducing agent and hydrazinium sulfate as a stablizer. The effects of injection volume, coil length and reagent concentration on optimum sensitivity are evaluated.
Linares, et al., Anal. Chem., 58, 120-124 (1986) discloses analysis of binary and tenary mixtures of arsenite, arsenate and phosphate using an injection valve; mixes molybdate and ascorbic acid prior to their confluence with the sample, which is said to yield higher peaks than two sequential confluence; and uses nitric acid instead of sulfuric acid.
Johnson and Petty, Anal. Chem., 54, 1185-1187 (1982) disclose a reverse flow injection analysis method in which a mixture of reagents (sulfuric acid, molybdate, ascorbic acid) are added to a sample stream; but only orthophosphate, apparently is analyzed.